Photopolymerizable (photohardenable) compositions and films containing binder, monomer, initiator and chain transfer agent are described in the prior art and sold commercially. One important application of photopolymerizable layers is in graphic arts. Photopolymerizable layers on conductive supports currently may be used as electrostatic masters for analog color proofing and are considered as promising future materials to be developed for digital color proofing applications. For the analog color proofing application, a photopolymer layer is coated on an electrically conductive substrate and contact exposed with an ultraviolet (UV) source through a half tone color separation negative. The photopolymer hardens in the areas exposed with an ultraviolet source due to polymerization and remains in a softer state elsewhere. The differences in viscosity between the exposed and unexposed areas are apparent in the transport properties, i.e., the unexposed photopolymer conducts more electrostatic charge while the UV exposed areas are substantially nonconductive. By subjecting the exposed photopolymer layer to a corona discharge a latent electrostatic image is obtained consisting of electostatic charge remaining only in the nonconducting or exposed areas of the photopolymer layer. This latent image can then be developed by application of a liquid electrostatic toner to the surface. When the developer has the opposite charge as the corona charge, the developer selectively adheres to the exposed or polymerized areas of the photopolymer layer. The developer in the exposed areas subsequently can be transferred to a surface such as paper to produce an image. The photohardenable electrostatic master described is negative working. Color proofs can be prepared by repeating the procedure using other photopolymer masters, imagewise exposing each master through a different color transparency, corona charging, developing each exposed master with a correspondingly colored developer, and transferring each developed image sequentially in register on the preceding transferred image.
Dueber U.S. Pat. No. 4,162,162 describes certain photopolymerizable compositions which in addition to the basic constituents for a photohardenable composition contain a visible sensitizer and a photoinhibitor. Such a composition is used primarily for preparation of lithographic printing plates. Dueber does also disclose the preparation of colored images from color separation negatives suitable for color-proofing. The Dueber invention is directed to photopolymerizable compositions which are developed by conventional means and not by an electrostatic process. Although various photopolymerizable compositions are disclosed by other inventors to be useful in electrophotography, photopolymer electrostatic masters capable of duplicating the image characteristics of a printing press are only now being developed. It is desirable to provide positive or negative images from a single photopolymer electrostatic master since such a master will satisfy the proofing needs of all printers whether they use negative or positive color separations.
It has now been found that negative or positive images can be provided which are substantially duplicates of a printing press by exposure means described more fully below utilizing a photohardenable electrostatic master having a layer containing a polymer binder, ethylenically unsaturated compound, photoinitiator, photoinhibitor and at least one visible light sensitizer.